control-flow.md

Control Flow

Control Flow primitives

• if expressions
• loop and while loops
• match expressions
• for loops
• break and continue
• return and ?

Using if as a statement

• Tests if a boolean expression is true
• Parentheses around the conditional are not necessary
• Blocks need brackets, no shorthand

fn main() {
    if 1 == 2 {
        println!("integers are broken");
    } else if 'a' == 'b' {
        println!("characters are broken");
    } else {
        println!("that's what I thought");
    }
}

Using if as an expression

• Every block is an expression
• Note the final ; to terminate the let statement.

fn main() {
    let x = if 1 == 2 {
        100
    } else if 'a' == 'b' {
        200
    } else {
        300
    };
}

Using if as the final expression

Now the if expression is the result of the function:

fn some_function() -> i32 {
    if 1 == 2 {
        100
    } else if 'a' == 'b' {
        200
    } else {
        300
    }
}

Looping with loop

loop is used for (potentially) infinite loops

fn main() {
    let mut i = 0;
    loop {
        i += 1;
        if i > 100 { break; }
    }
}

Looping with loop

loop blocks are also expressions...

fn main() {
    let mut i = 0;
    let loop_result = loop {
        i += 1;
        if i > 10 { break 6; }
        println!("i = {}", i);
    };
    println!("loop_result = {}", loop_result);
}

while

• while is used for conditional loops.
• Loops while the boolean expression is true

fn main() {
    let mut i = 0;
    while i < 10 {
        i += 1;
        println!("i = {}", i);
    }
}

Control Flow with match

• The match keyword does pattern matching
• You can use it a bit like an if/else if/else expression
• The first arm to match, wins
• _ means match anything

    fn main() {
        let a = 4;
        match a % 3 {
            0 => { println!("divisible by 3") }
            _ => { println!("not divisible by 3") }
        }
    }

for loops

• for is used for iteration
• Here 0..10 creates a Range, which you can iterate

fn main() {
    for num in 0..10 {
        println!("{}", num);
    }
}

for loops

Lots of things are iterable

fn main() {
    for ch in "Hello".chars() {
        println!("{}", ch);
    }
}

for under the hood

• What Rust actually does is more like...
• (More on this in the section on Iterators)

fn main() {
    let mut iter = "Hello".chars().into_iter();
    loop {
        match iter.next() {
            Some(ch) => println!("{}", ch),
            None => break,
        }
    }
}

Break labels

If you have nested loops, you can label them to indicate which one you want to break out of.

fn main() {
    'cols: for x in 0..5 {
        'rows: for y in 0..5 {
            println!("x = {}, y = {}", x, y);
            if x + y >= 6 {
                break 'cols;
            }
        }
    }
}

Continue

Means go around the loop again, rather than break out of the loop

fn main() {
    'cols: for x in 0..5 {
        'rows: for y in 0..5 {
            println!("x = {}, y = {}", x, y);
            if x + y >= 4 {
                continue 'cols;
            }
        }
    }
}

return

• return can be used for early returns
• The result of the last expression of a function is always returned

fn get_number(x: bool) -> i32 {
    if x {
        return 42;
    }
    -1
}